It has long been considered that the human knee is particularly susceptible to sudden and debilitating injury. The knee structure is complex and has little resistance to external forces applied from nonaxial directions. Further, the knee is frequently unable to move effectively to absorb such nonaxial forces, since placing of the foot fixes the distal end of the leg and effectively converts the leg into a rigid beam with the knee as the "weak spot".
In addition, the activities that many people engage in frequently expose the knee to external forces under conditions where the knee is already under stress, thus increasing the severity of injuries. It is unfortunately common for severe knee injuries to be incurred by those who participate in sports such as football, skiing and soccer, where the legs are subject to a highly stressed environment. Such injuries are often sufficiently severe that extensive reconstructive surgery is required. Even with surgery it is not uncommon for these injuries to force the end of an athlete's career or to limit the ability of the injured person again to participate in recreational athletics. In addition, a knee injury even for a non-athlete can result in permanent limitation of knee function and a greater or lesser degree of impairment of the person's leg function.
The knee is a complex joint, so there are several components which are of major importance in assessing injuries of the knee; these will be described in detail below in connection with FIGS. 1 and 2. Because one or more of these components may be hyperextended, dislocated or severed a variety of different knee injuries resulting from traumas or other applied forces are possible. While there have been efforts in the past to analyze the response of the knee to such forces, it has only been possible to determine gross response for the knee as a whole; analysis of the response of individual components on a quantitative basis could not be performed. There have been attempts to simulate knees with anthropomorphic leg devices, but these prior art devices have not been capable of determining the individual stresses that each of the knee-related components, such as ligaments and muscle groups, is subject to under a particular strength and direction of force. The prior art devices have essentially all treated the knee as a simple unitary hinged joint for which the external axial loading has been measured, but no device has had the capability of determining the internal effects resulting from the external forces. In a few cases, such as the device shown in U.S. Pat. No. 4,349,339, provision has been made for measurement of forces in three dimensions. However, such a device is still capable only of measuring gross external shearing forces in three dimensions since the knee is still treated as a simple hinge. Thus while one can determine lateral or anterior/posterior externally applied forces, the prior art devices cannot provide any information with respect to the effects of such forces on the individual components of the knee or of the type and severity of the knee injury which would occur under such forces.
It would be particularly advantageous to be able to make reproducible and quantitative studies of the responses of individual knee components to external stresses, and to have a device which would be capable of accurately determining such responses of the individual principal components of the knee when the knee as a whole suffers externally applied forces, such as a blow or similar trauma. Such information would, for instance, allow the design of knee braces and other orthopedic devices which would maximize the amount of protection available for the types of traumas most likely to be encountered in sports. Similarly, with such information physicians would be able to predict the most likely type of injuries to be sustained under defined external circumstances and therefore be able to evaluate actual knee injuries more quickly and efficiently. Equipment designers, such as automobile designers, could also design their products so as to minimize the possibility of the equipment causing major trauma to persons' knees in a collision or other accident.
It would also be advantageous to have a method and device for use in teaching and study which would give accurate and reproducible data on the pathology of the knee. This would be valuable for general medical training as well as to familiarize emergency medical and physical training people with knee injuries and disorders.
Finally, it would be valuable to have a method and device for obtaining consistent and reproducible quantitative data on the knee and its components, so that knee testing instruments and orthopedic devices could be evaluated against consistent quantitative standards.